1. Field of the Invention
This invention relates to drive sprockets of the type used with snowmobiles. In particular, the invention relates to a drive sprocket for use with snowmobile tracks that have open windows.
2. Description of the Related Art
Snowmobiles are becoming increasingly popular for travel over snow, ice, and other surfaces. In addition, demand is increasing for faster and more powerful snowmobiles. This demand for more speed and power is particularly evident in the sport of snowmobile racing, although the demand also exists in the general market.
A snowmobile uses a drive train with a track for propulsion. The track is in contact with the surface over which the snowmobile runs. Drive sprockets propel the track, which in turn propels the snowmobile.
In conventional snowmobiles, four to six drive sprockets are required to propel the track. These sprockets have involute (i.e., internal) drive lugs extending from the sprocket's sides. The drive lugs push against knobs on the inside surface of the track. This arrangement is adequate for use at lower speeds and powers. However, as speed and power increase, the drive lugs can slip or “ratchet” over the knobs, resulting in a loss of power transferred through the track to the surface over which the snowmobile is operating.
One proposed solution to the problem of slippage is to use a sprocket with convolute (i.e., external) drive lugs. This sprocket has an appearance somewhat akin to a gear. The convolute lugs fit into windows (i.e., hole.) in the track. When the sprocket turns, it pushes against the front edges of the windows, driving the track. The spans of track between the windows can be covered in steel or other reinforcing material so as to withstand the driving force better. However, while this convolute design is superior to the involute design, slippage can still occur at higher speeds and powers. In addition, four to six drive sprockets are still typically required.
Yet another proposed solution is to use a sprocket with involute lugs on one side in addition to convolute lugs. This type of sprocket is known as an “extrovert” sprocket. Unfortunately, when the involute lugs drive the track, all of the applied force comes from one side of the sprocket. As a result, these types of sprockets can deform, again leading to slippage problems.
Finally, some existing snowmobiles use a combination of involute and convolute drive sprockets. With this arrangement, at least four sprockets are still needed. Furthermore, the two different types of sprockets drive different types of structures in the track. As a result, synchronizing the sprockets to apply force evenly is problematic.